Zener diode reference voltage standards

ABSTRACT

A method of operating a voltage reference element such as a zener diode comprises applying at least two current values to the device in respective periods of time one said value being such as to provide desired reference voltage characteristics of the device and the other being such that the average current during both periods provides a selected power dissipation to set a required temperature of operation of the device.

BACKGROUND OF THE INVENTION

This invention concerns the operation of Voltage references dependent onthe “Zener” or “Avalanche” characteristics of a semiconductor diodecommonly referred to by those versed in the art as “Zeners”, ZenerDiodes or Zener References. This type of semiconductor device produces arelatively precise voltage across its cathode and anode for a range ofcurrents passing through it in the reverse mode, that is the oppositedirection, Cathode to Anode to that which produces normal diode functionbehaviour. For certain types of these diodes extremely stable voltagebehaviour is realisable where the reverse current is set to a suitableand stable value.

It is one of the prime objectives of those making stable voltagereference standards based on the principle to minimise the Very LowFrequency (VLF) noise and long term random instability of outputVoltage. It is a further objective to minimise the output voltagedependence on external. environmental conditions particularly variationsin temperature and atmospheric pressure.

BRIEF SUMMARY OF THE INVENTION

It is generally known that random noise and instability generated by theZener diode is reduced by increasing the junction area of the diode.However, this can further be improved by operating the Zener at anoptimum current density which reduces the noise but, in a large areadiode, can dissipate sufficient power to cause the Zener and itspackaging to rise to such high temperature that oven temperature controlbecomes difficult or impossible without compromising the long termvoltage stability of the Zener.

It is accordingly an object of the invention to provide means to operatea Zener diode reference of large junction area at an optimal currentdensity whilst maintaining or controlling the temperature of the siliconchip on which the diode is diffused at a lower increment above theambient temperature than would have prevailed without application of theinvention.

The invention is illustrated by way of example in the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a, 1 b and 1 c are schematic diagrams of known arrangements.

FIG. 2a illustrates the principle of operation of the invention withFIG. 2b showing the current waveform with two current periods.

FIG. 3 illustrates the principle of the invention with a loop controlledsecond current period.

The arrangements known in the prior art include those of FIGS. 1a, 1 band 1 c.

FIGS. 1a shows the schematic of a type of reference element thatincorporates a Zener diode, 1, and a transistor, 2, in one thermalenvironment, 3, commonly a single silicon chip packaged in standardsemiconductor device packaging well known to those versed in the art. Inthis example advantage is gained from using the transistor base toemitter voltage which is a voltage which reduces with increasingtemperature, to add to the Zener voltage which increases with increasingtemperature. This is known as a compensated Zener or a ReferenceAmplifier. A current, which is derived from circuiting coupled to thetransistor in known manner but which for clarity is not shown in this orsubsequent drawings, is passed through the transistor to bias it and thesame or different current through the Zener, these currents being chosensuch that the temperature coefficient of voltage of the output, which isthe sum of the Zener voltage and the transistor base emitter voltage, isnominally zero.

In the illustration of FIG. 1b, a temperature sensor such as athermistor, 5, and external oven, 4, is added in close thermal contactwith the Zener to control the temperature of the simple embodiment ofFIG. 1a, thus further reducing the effective temperature coefficient butnecessarily resulting in a higher temperature of operation of theSilicon junctions unless cooling is used.

In the illustration of FIG. 1c, a further transistor, 7, is included tosense the temperature of the silicon chip and a heating element, 6, isdiffused into the chip to allow its temperature to be adjusted. It isthen a relatively simple matter for those versed in the art to use thetransistor temperature sensor and the heater to control the temperatureto a high degree of constancy.

It should be apparent that to provide a reasonable degree of control ofchip temperature over varying ambient temperature then the arrangementsof FIGS. 1b and 1 c require that the silicon chip is operated at asignificantly higher temperature than that which results from thecircuit of FIG. 1a and that this in turn limits the magnitude of biascurrent through the Zener diode that can be chosen because of the powerdissipation and self heating that results.

An arrangement in accordance with the invention and shown in FIG. 2aallows operation of the Zener diode at optimal current density bypulsing the bias current though it at a value equal or similar to theoptimal current density and thus giving two or more distinct periods ofoperation which would normally, but not necessarily, be repeatedcontinuously.

During the first period, t₁ a precisely defined current, I_(b1) ispassed through the Zener diode, 1, which may be a simple Zener diode asshown in FIG. 2 or a reference element similar to that of FIG. 1a andthe resulting output voltage sampled and stored on the capacitor of theSample and Hold or Track and Hold circuit, 14, being sampled duringperiod t₁, 13, this being a well known technique for storing voltagevalues commonly used by those concerned with the design of Analogue toDigital Converters. I_(b1) is the optimum bias current, 8, chosen tominimise the Random noise in the Zener, 1, and is typically too high forsatisfactory continuous application. I_(b1) is therefore turned off orreduced during a second period such that I_(b2), a typically differentcurrent, 9, then flows through the Zener. This operation is symbolisedby switch, 10, shown connected to I_(b1), for period t₁, 11, and toI_(b2) for period t₂, 12.

The value of I_(b2) and the periods t₁ and t₂ for which I₁ and I_(b2)respectively flow can thus be chosen so that the average current in theZener provides an acceptable level of self heating where the totalperiod t₁ plus t₂ is significantly faster than the thermal time constant(a measure of the speed of heating and cooling) of the Zener. A typicalthermal time constant for this type of component is many tens of secondsso if the period t₁+t₂ is much less, say of the order of tens ofmilliseconds, temperature fluctuations during the sample time t₁ will benegligible and repeated sampling will give a steady output voltage shownon output terminals, 15, and 16. This output value will have less LowFrequency random voltage noise and instability because it is sampled athigher bias current than would be the case if it was measuredcontinuously at lower bias current. It should be noted that pulsetesting of electronic components, where test currents are pulsed on forthe duration of the test but otherwise off is well known in the priorart. However, the object of this invention is to operate normally inthis manner and to provide a second level of current I_(b2) which can bechosen to give a specific degree of self heating or can be controlled toset a particular temperature of the Zener reference silicon chip andwould not normally be zero or merely turned off. FIG. 2b is a simplegraph showing the resulting current waveform with I₂ set for aparticular level of power dissipation in the Zener. In practice this canbe varied whilst leaving I_(b1) and hence the output voltage at aconstant value.

A more useful and sophisticated embodiment of the invention is shown inFIG. 3 where a Zener reference element as before, 1,2,3, is biasedduring time t₁ with current I_(b1) as before but where I_(b2) isreplaced, during period t₂ with a current supplied by resistor, 19, andamplifier, 18. In this case the desired Zener voltage is sampled asbefore but also the base to emitter voltage (Vbe) of the transistor issampled during period t₁ in a second sample and hold or track and hold,17, to give a measure of the temperature of the silicon chip and thus ofthe components of the reference element. This sampled, temperaturedependent, voltage is then used in a control loop by connecting toamplifier, 18, to control the magnitude of current through the-resistor,19, during the second period t₂. It would also be possible to adjust theduration of the period t₂ with respect to period t₁, or to adjust boththe magnitude of current and the relative period, but in either case theaverage sampled base emitter voltage Vbe and hence the chip temperature,Tc, is maintained at a constant value.

It should be appreciated that there are many variations to this designpossible and that they may depend on the structure of the referencechosen. In particular, a third period of time may be included to allowtemperature measurement, for example by reversing the Zener diode andmeasuring its forward diode voltage. It is also possible to leave I_(b1)flowing continuously whilst making I_(b2) add or subtract to it duringthe second period t₂.

What is claimed is:
 1. A method for providing bias current to andsensing the voltage of a Zener reference diode such that at least twocurrent values are applied occurring in at least two periods of time oneof such values being selected for desired Zener referencecharacteristics and during which the Zener voltage is sampled ormeasured and the other being chosen such that the average current duringboth periods provides a selected degree of power dissipation to set arequired temperature of operation of the Zener diode.
 2. A methodaccording to claim 1 where the relative duration of the two said periodsis adjusted and chosen such that the average current during both periodsprovides a selected degree of power dissipation to set a requiredtemperature of operation of the Zener diode.
 3. A method according toclaim 1 or 2 where the Zener reference diode comprises a silicon chip onwhich a Zener or avalanche diode is diffused together with a temperaturecompensation transistor or temperature compensation diode.
 4. A methodaccording to claim 1, 2 or 3 where the temperature sensor is alsointegrated on to the said silicon chip or is the said compensationtransistor or diode or is the said Zener diode connected in forward biasmode for a period of time in order to sense the temperature.
 5. A methodaccording to claim 3 or 4 where the said adjusted second bias current oraverage current is controlled to maintain constant or near constantoutput from said temperature sensor regardless of chances in ambienttemperature.
 6. A method according to claims 3, 4, or 5 where a thirdperiod is used to measure or sample said sensed value of temperature.